Holiday online sales remain strong

NEW YORK (CNNMoney.com) -- Online retailers continued to post big holiday sales numbers last week, with a jumpstart from a record-setting Cyber Monday, according to data released Wednesday.

So far, more than $17.5 billion has been spent online in the first 35 days of the holiday shopping season through Dec. 5, a 12% increase over the same time last year, according to comScore, an online analytics firm.

For the week starting with Cyber Monday, four separate days topped $800 million in aggregate sales for online retailers.

Shoppers kicked off the week by spending $1.03 billion on Cyber Monday, which became the heaviest online spending day on record. That was followed by $911 million on Tuesday, $868 million on Wednesday and $850 million on Thursday.

Overall, sales for the week ended Dec. 5 were up 9% from the year-earlier period -- a little below the rate of growth for other parts of the season.

"We anticipated that the post-Cyber Monday period would experience a slight hangover after many of the retailers' most aggressive deals and promotions expired," comScore chairman Gian Fulgoni said in a statement.

Promotions such as free shipping increased in popularity this year, with each of the most recent three weeks seeing more than 50% of all transactions including the incentive.

"Without a doubt, free shipping has become a critical driver of e-commerce purchasing, with the majority of consumers indicating that they will abandon their shopping carts if they get to check-out and find that free shipping is not included," Fulgoni said.

Online retailers might have even more to anticipate. Fulgoni said he expects to see activity continue to pick up in the middle of December, when online buying typically peaks.